Genetic Variation I

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Transcript Genetic Variation I

Genetic Inheritance & Variation
• No 2 organisms in a species are the same
(except “clones” or monozygotic twins)
• Genetic variation is essential for evolution
and change to occur
• There are 2 main processes that generate
– Mutation
– Recombination
Mutation and Recombination
• Mutation is a change in the genetic
• Recombination is a different arrangement of
the same genetic material
• The cat sat on the mat
• The cat sat on the hat - mutation
• The mat the cat sat on - recombination
• First of all, we need to look at genetic
Mendel’s experiments
• Gregor Mendel (a 19th century Czech
monk) worked out the basic laws of genetic
inheritance by breeding pea plants
• He chose simple characteristics that are
determined by single genes (monogenic)
• Many characters such as height, IQ, disease
susceptibility are determined by several
genes (polygenic)
Mendel’s first cross
P1 (parental) generation: wrinkled seeds
crossed with smooth seeds
F1 generation: all smooth seeds. Crossed
with itself………...
F2 generation: smooth and wrinkled
in ratio 3:1
Mendel’s genetic hypothesis
Genes come in pairs. Each of the parents has
2 copies of this gene. The “A” form gives smooth
seeds, the “a” form gives wrinkled.
Parents produce gametes (eggs, pollen)
which have 1 copy of the gene.
Fertilisation produces the F1 generation, all smooth
because the “A” form is dominant over “a”;
“a” is recessive
Each F1 plant produces equal numbers of A and a
gametes which fertilise at random to produce the F2
plants. 1/4 of them are AA (smooth), 1/2 are Aa
(smooth) and 1/4 are aa (wrinkled).
Cross with two genes
AB Ab aB ab
4 types of gametes
in equal numbers
9/16 yellow/smooth
3/16 green/smooth
3/16 yellow/wrinkled
1/16 green/wrinkled
Summary of Mendel’s experiments
• Genes in an organism come in pairs
• Some forms (“alleles”) of a gene are dominant
over other alleles which are recessive
• One (at random) of each pair of genes goes into a
gamete (segregation)
• Gametes meet randomly and fertilise
• The numbers and types of offspring in a cross are
determined by the above laws
• Separate genes behave independently of each other
(later, exceptions to this rule were found)
Genes and chromosomes
• Genes can have several different forms due to
mutations in DNA sequence. These forms are
called alleles. Property of having different forms is
called polymorphism
• Normal human body cells (“somatic” cells) are
diploid: 23 pairs of chromosomes:
– Numbers 1-22 (autosomes)
– X and Y (sex chromosomes)
– XX in females, XY in males
• Gametes (eggs, sperm, pollen) are haploid, i.e.
they have a single copy of each chromosome
Autosomal dominant inheritance
Person with trait in each generation
Males and females equally likely
to show trait
Where 1 parent is heterozygous,
about 50% of offspring show trait
Example: Huntington’s disease
Autosomal recessive inheritance
•Trait may “skip” generations
•Males and females equally likely to show trait
•Heterozygotes (“carriers”) do not show trait
•About 25% of offspring of 2 carriers will show trait
•Example: cystic fibrosis
X-linked recessive inheritance
Carrier (heterozygous,
unaffected) mothers pass the trait
to about 50% of sons
Trait is never transmitted
from father to son
In the population, trait will be much more common in males
than females. Example: muscular dystrophy
Jumping genes
• Genomes are not always stable. Some DNA sequences can
jump from one place to another (transposons)
• Transposons can be responsible for things like antibiotic
resistance in bacteria
• They can also affect the expression of a gene near to where
they jump
• If a transposon jumps in some cells but not others, can get a
variegated phenotype
Maize (corn) cob
Transposon mechanism